Geochemical Evidence of the Sebree Trough During the Late Ordovician: Chemostratigraphy of the Point Pleasant-Utica intervals: A Talk by Dr. Julie Bloxson. Summary & Review. Presented for The Pittsburgh Association of Petroleum Geologists meeting, January 15, 2026

     I missed the beginning of this lecture, but found Dr. Bloxson’s November 2022 paper in the Ohio Journal of Science to supplement. She talked a bit about ‘peripheral forebulge migration’ across foreland basins. Bathymetric lows in marine environments are where the deposition of mudstones occurs. Structures control deposition, including reactivated structures. These structures can lead to deposition above storm wave base. She asks: “How does Utica/Point Pleasant appear throughout Ohio in terms of facies and bulk mineralogy?” and “How does Sebree Trough affect Utica/Point Pleasant?” The Utica and Cincinnatian formations are top seals.

   

     In her paper, she explains the influence of structures, both topographic lows and topographic highs, in controlling deposition:

“This study of complexly intertonguing Upper Ordovician (Katian; upper Mohawkian–lower Cincinnatian) carbonates and shales demonstrates the utility of electrofacies in discriminating subtle differences in lithology that reflect noticeable changes in structure, thickness, and facies within carbonate formations (Trenton and Lexington limestones) and overlying shale formations (Point Pleasant Formation and Utica shale). Changes on relatively large scales, such as the thinning of carbonates and thickening of the shales across the center of the state, reflect the presence of the Sebree Trough (Figs. 8, 9, and 10). Smaller-scale variations in carbonate and argillaceous content across areas like the Waverly Arch and several smaller fault zones (Fig. 11) are apparently heterogeneities in these formations that suggest structural controls on deposition.”

     These were warm seas, tropical, during deposition. Volcanic activity is documented all over eastern North America by abundant k-bentonite beds. There are more fossils in the Lexington/Trenton carbonates than in the Point Pleasant-Utica mudstones. Storm beds are present more in the Point Pleasant, which is richer in mudstone and siliciclastic deposits. It is often a laminar black mudstone. Brown units indicate more clay minerals. Calcareous-rich storm deposits are present in Western Ohio. Some aerial exposure is documented there. Cores show that Utica-Pont Pleasant is very heterogeneous.

     The Sebree Trough is a major controlling bathymetric low feature. Reactivation of basement structures is involved as well, including the Reelfoot Rift to the south, which allows the development of cold phosphatic waters.

     “What controls Utica/Point Pleasant variability?” Mapping Utica-Point Pleasant and Trenton-Lexington as one unit each was done. Cluster analysis was used on cores. Well log analysis includes tabulating shale vs. non-shale components. Cluster analysis can distinguish different types of shale, including high TOC shale. GAMLS, a pattern recognition software, was used to differentiate different responses and was then compared to cores. Ten different facies were identified via logs. GMALS analysis compared favorably to cores. Calcite content is a big factor. Logs may need centimeter-scale resolution, which is often not available as it is with visual analysis of cores.

     Seeking geochemical evidence that the Sebree Trough affected deposition of anoxic high TOC mudstones in Utica-Point Pleasant involves detrital indicators, carbonate and phosphate indicators, and redox-sensitive trace metals. These were used to develop the chemo-facies.

     Chemo-facies 1 and 2 are anoxic shales on a scale of anoxic to oxic. Chemo-facies 3 is mostly found in the Farley core in Washington County.  She notes that its interpretation is difficult. The Portage County core is interpreted as being in the Sebree Trough extension. The Farley core was interpreted as open-marine and mid-ramp. The Tracker core (Portage County) found uranium in shale, but the Farley core did not. There were likely frequent storms in the Farley area, which added oxygen to the bottom waters, at least temporarily. Diagenetic reorganization could have happened as well. Facies is often related to calcite content. The Sebree Trough likely extends into Ohio and connects with the Appalachian basin.

 

Q&A

Can argillaceous limestone generate hydrocarbons? Yes, as long as TOC is present in high enough amounts.

     Dr. Bloxson's abstract for her talk is below:

     More maps and tables from the Ohio Journal of Science Paper are shown below.  

References:

 

Geochemical Evidence of the Sebree Trough During the Late Ordovician: Chemostratigraphy of the Point Pleasant-Utica intervals: A Talk by Dr. Julie Bloxson. Presented for The Pittsburgh Association of Petroleum Geologists meeting. January 15, 2026.

Subsurface Relationships between the Sebree Trough and Carbonate-Siliciclastic Mixing in the Upper Ordovician Lexington-Trenton and Point Pleasant-Utica Intervals in Ohio, USA, using Multivariate Statistical Well Log Analysis. Julie M. Bloxson, Beverly Z. Saylor, and Frank R. Ettensohn. Ohio Journal of Science. 122(2):52-74. November 2022. ojs-3897-bloxson.pdf